It has always been a priority for signal intelligence to be able to detect the presence of new energy from emitters and to obtain not only the azimuth in terms of the line of bearing (LOB) to the emitter but also elevation of the incoming signal so as to be able to locate the emitter.
In so doing, one can ascertain the position of the emitter and its distance from the detector array, thereby to permit deploying countermeasures.
In the past, namely in the 1960s and '70s, the requirement was to be able to detect and classify as many as 10 new energy sources per second. However, with the proliferation of emitters, it has now become a requirement to be able to detect and direction-find on as many as 1,000 emitters per second. This has proved to be a troublesome requirement, especially with prior architectures. These legacy systems would first have to ascertain whether there was a signal worth direction finding on. This was accomplished by providing a new energy alarm (NEA) that alerted the direction finding system of the existence and frequency of an emitter that had popped up. The DF processing then worked through the detected new energy sources as fast as it could.
However, there was no indication prior to instigating the direction finding process which signals were important to direction-find on. The prior systems therefore had to simply jump in and try to direction-find based on whatever order the new energy emitters came in, with no prioritization.
Moreover, a more important problem was that the emitters could come on briefly and then disappear. As will be appreciated, in prior DF systems it was not possible to direction-find on a signal that had disappeared. It was therefore necessary to immediately perform direction-finding on any and all new energy, without knowing whether the emission was important or not.
Moreover, the prior DF systems were based on narrow band receivers, which never were able to keep up with all of the new energy alarms that were coming in. With the present proliferation of emitters, the older narrow bandwidth systems are not able to keep up with even a significant percentage of the load.
The only way that one could potentially have accommodated all of the new sources would be to duplicate the direction finding systems. Thus, if one had 100 complete direction finding systems that could perform 10 direction finding operations per second, one would be able to accommodate the requirement of direction finding on 1,000 emitters per second. However, by merely duplicating the number of full-up direction finding systems, one would have the complication of coordinating between the direction finders. Moreover, aside from the coordination problem, one would require extra sets of hardware, involving more power, more heat, more cooling, increased weight, increased physical size, and increased cost.
Moreover, one would have to use the same coherent antenna array. However, if one divides the available power at the antenna elements between all of the direction finding systems, one loses power per channel. This means that the ability to direction-find on a low-level signal, which is always a problem, gets even more exacerbated.
Thus, there did not appear to be a solution to the requirement of being able to direction-find on two orders of magnitude more signals using the prior narrow band directed DFing systems.
By way of background, the narrow band systems employ a number of narrow band receivers, which for HF requires 3 to 6 KHz of bandwidth. For VHF/UHF, bandwidth is on the order of 25 to 100 KHz.
In order to be able to tune through the various bandwidths, one had to have very fast-tuned receivers. In order to capture all sources that pop up, one would then have to quickly tune the whole array's worth of receivers to whatever emitter was providing the new energy alarm (NEA).
For instance, in HF, if the new source was determined to be at 1 MHz, all of the narrow band receivers would have to be tuned to 1 MHz. For a 16-element array this meant that all 16 receivers would have to be tuned to 1 MHz. Thereafter, data was collected for some period of time, whereupon the next new source, which could conceivably be at 2 MHz, would require tuning the entire array of receivers to another frequency, for instance 2 MHz, before performing a direction finding on the identified signal. Because new energy sources appear at a very fast rate, the receivers would be constantly tuning to some frequency, meaning that the receivers would be tuning all over the place. Moreover, the prior narrow band finding scheme could only service the new energy sources as they came in and direction-find on them as best as they could. This was done without identifying whether they were a threat or of interest.
Thus there was no mechanism to select amongst those signals that triggered a new energy alarm. The result was that the direction finding system merely took the next new source in the queue and processed it.
While it was possible for an operator to exclude certain frequency ranges to focus the direction finding process, by so doing the operator would be simply throwing out processing in the excluded range thereby allowing the direction finding system to process signals in other frequency ranges. Thus, once the decision was made to exclude a band, there was no way of using the results from the processing of other bands to obtain results for the excluded band.
In operation, the narrow band systems coherently sample the antenna array so that in addition to obtaining a line of bearing or azimuth, the elevation (or depression) of the signal can be calculated allowing the system to accurately process sky waves. Not only did one wish to ascertain the angle at which the signal came in, the elevation angle would provide some indication of range to the emitter.
In one embodiment of the narrow band systems, the array was coherently sampled for a 16-element array, and 16 receivers were provided. All of these receivers would feed their own analog-to-digital converters. The analog-to-digital converters would then have their outputs processed by a Fast Fourier Transform (FFT), so that the time domain information would be converted to frequency domain information. The result was amplitude and phase information that was correlated against an existing direction finding database, sometimes called a manifold, to compute angle of arrival and elevation.
It will be appreciated that after a new energy alarm one had to tune each receiver, one receiver per antenna element, and to perform an FFT on the output of each antenna element's receiver to provide for the frequency domain values used by conventional direction finding techniques. Thus the FFT was done only after a signal was detected as being up. This was done seriatum, one signal source at a time, and required that the signal be up when performing the direction finding function.
In the narrow band receivers one of course did not have to pick up the entire bandwidth of the signal. Depending on the modulation type, if amplitude modulated, one would not want to look at the entire 25 KHz bandwidth. The prior systems thus picked out a very small slice, perhaps 100 Hz or down to 10 Hz, depending on how important the signal was, in order to be able to pull out the line of bearing from the amplitude and phase value of the one bin that was available.
Note that the narrow band systems did not wish to listen to the entire signal, especially, for instance, an FM stereo station that is 200 KHz wide, but rather wished only to focus on finding the direction of arrival. In so doing, one could pick out just 25 KHz and do a direction finding operation on just that slice of energy.
In summary, upon ascertaining of a new source, one performed a direction finding process on the frequency of interest, after which the system would then go and service the next new source. The result of the use of narrow band assets was that a tremendous amount of tuning was involved, with signals popping up all over the place such that one would never know if one was going to be able to service all of the new sources.
Thus the narrow band systems first had to ascertain that new energy was coming in and then quickly tune the receivers so as to be able to obtain the amplitude and phase information after having processed the signals by a Fast Fourier Transform.
With respect to the deficiencies in the prior art narrow band systems, the major deficiency was the inability to keep up with all of the new energy alarms. For instance, it is not the fact that all 10 signals might be above the signal-to-noise ratio that one is concerned with; it is that there are other signals that are below it. Thus, if the requirement was to process 10 new energy alarms per second, if the signal-to-noise ratio for new sources was low, one might only be able to measure 5 sources per second. As a result, the prior narrow band systems designed to track 10 sources per second might not be able to do so for weak signals.
Secondly, one of the problems with the above narrow band systems was that one did not know the priority of the signal being processed. Thus the direction finding system could be measuring and computing the angle of arrival for a signal of low priority while a high priority signal went un-serviced.
The net result of the prior systems was that they missed processing emitters that had been previously identified as being above a certain alarm threshold. Moreover, many emitters dropped out before they could be processed. The problem was so severe that with proliferating emitters, one would be lucky to obtain direction finding results for 10% of the signals.